| .M | MathWorks | Fichiers de développeurs | MATLAB Source Code File | FMPMD2000 | Unknown Developer |
| .M2 | Unknown Developer | Fichiers audio | PC-98 Game Music File | FMPMD2000 | Unknown Developer |
| .M3K | Unknown Developer | Fichiers compressés | Interactive Digital Content File | Unknown Software | Unknown Developer |
| .M3V | Unknown Developer | Fichiers audio | DJ Mix Pro Playlist | DJ Mix Pro | Unknown Developer |
| .M5D | Unknown Developer | Fichiers divers | Unknown File Type | Unknown Software | Unknown Developer |
| .M9D | Unknown Developer | Fichiers divers | Unknown File Type | Unknown Software | Unknown Developer |
| .MAC2 | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MACRO | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MAILBOX | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MAPS | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MARIO | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MAU | Unknown Developer | Fichiers divers | Media Attachment Unit | Unknown Software | Unknown Developer |
| .MBL | Vernier Software & Technology | Fichiers de données | Logger Pro Data File | Multi-Batch | Unknown Developer |
| .MBZ | Unknown Developer | Fichiers de plugin | Pogoshell NES Emulator Plugin | PogoShell | Unknown Developer |
| .MCH | Interscape | Fichiers de paramètres | Machinery HDR Effects | Unknown Software | Unknown Developer |
| .MCV | Visicom Media Inc | Fichiers de paramètres | ManyCam Effect File | Unknown Software | Unknown Developer |
| .MC_ | Unknown Developer | Fichiers compressés | Musicato Compressed File | Unknown Software | Unknown Developer |
| .MD | Various Developers | Fichiers de développeurs | Markdown Documentation File | mdcd10.arc | Unknown Developer |
| .MDL | MathWorks | Fichiers de données | MathWorks Simulink Model | mdl2obj tool | Unknown Developer |
| .MD_ | Unknown Developer | Fichiers de paramètres | Visc15 Adressen Setup File | Unknown Software | Unknown Developer |
| .ME | Unknown Developer | Fichiers divers | Temporary Delete Me File | Unknown Software | Unknown Developer |
| .ME1 | Unknown Developer | Fichiers de données | 21 Sample Winexam File | Unknown Software | Unknown Developer |
| .ME2 | Unknown Developer | Fichiers divers | Uncommon File Extension | Unknown Software | Unknown Developer |
| .MEI | Unknown Developer | Fichiers divers | Uncommon File Extension | Unknown Software | Unknown Developer |
| .MEK | Unknown Developer | Fichiers divers | Uncommon File Extension | Unknown Software | Unknown Developer |
| .MEMORY | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MERLIN | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MESSAGES | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .METAL | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MFC | Unknown Developer | Fichiers divers | Mankind File | Unknown Software | Unknown Developer |
| .MFO | Perl Express Group | Fichiers de données | Perl ModInfo Descriptor File | Unknown Software | Unknown Developer |
| .MGW | Unknown Developer | Fichiers de données | Microgateway Policy Implementation Data | Microgateway | Unknown Developer |
| .MHW | Unknown Developer | Fichiers divers | Unknown File Type | Unknown Software | Unknown Developer |
| .MICE | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MISC | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MJM | Unknown Developer | Fichiers de données | JDF Data Exchange Format | Unknown Software | Unknown Developer |
| .MK8 | Unknown Developer | Fichiers divers | Unknown File Type | Unknown Software | Unknown Developer |
| .MLI | Unknown Developer | Fichiers de données | Encyclopedia Media List | Unknown Software | Unknown Developer |
| .MLTV | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MMV | Sony | Fichiers vidéo | MicroMV Video File | Unknown Software | Unknown Developer |
| .MNW | Unknown Developer | Fichiers divers | Menu File | Unknown Software | Unknown Developer |
| .MODEM | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MOJ | Unknown Developer | Fichiers divers | Unknown File Type | Unknown Software | Unknown Developer |
| .MOUSE | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MP4S | Unknown Developer | Fichiers divers | Unknown File Type | Unknown Software | Unknown Developer |
| .MP6 | Unknown Developer | Fichiers de données | CS225 MP6 Huffman Encoding Text Data | Unknown Software | Unknown Developer |
| .MP_ | Open Source | Fichiers audio | Mobile Phone Sound File | P2k Commander | Unknown Developer |
| .MQF | Unknown Developer | Fichiers divers | Unknown File Type | Unknown Software | Unknown Developer |
| .MRA | Unknown Developer | Fichiers audio | VilagRadio Recorded Radio Program | Unknown Software | Unknown Developer |
| .MRS | MAIET Entertainment | Fichiers de jeu | GunZ Game Data File | MRS Decompiler | Unknown Developer |
| .MS-LOCKSCREENCOMPONENT-PRIMARY | Unknown Developer | Fichiers divers | Unknown File Type | Unknown Software | Unknown Developer |
| .MSA | Unknown Developer | Fichiers compressés | Magic Shadow Archiver Atari Disk Image | Magic Shadow Archiver | Unknown Developer |
| .MSB | Oracle Corporation | Fichiers de données | Oracle Binary Message File | Oracle .msb files unpacker | Unknown Developer |
| .MSEPUB | Unknown Developer | Fichiers divers | Unknown File Type | Unknown Software | Unknown Developer |
| .MSG2 | Apple | Fichiers de données | Golden Orchard Apple II CD Rom File | Unknown Software | Unknown Developer |
| .MSO | Microsoft Corporation | Fichiers de données | Microsoft Organization Chart Data File | Should Not Be Opened Manually | Unknown Developer |
| .MT9 | Electronics and Telecommunications Research Institute | Fichiers audio | Music 2.0 Song | Unknown Software | Unknown Developer |
| .MTC | Make The Cut, LLC | Fichiers de mise en page | Make The Cut! Project | Unknown Software | Unknown Developer |
| .MTV | MyMPX.org | Fichiers vidéo | MTV Video Format File | Unknown Software | Unknown Developer |
| .MUK | Unknown Developer | Fichiers audio | Multak Mini 8162 Encoded Audio Karaoke | Unknown Software | Unknown Developer |
| .MUV | Unknown Developer | Fichiers divers | Unknown File Type | Unknown Software | Unknown Developer |
| .MUZIIC | Unknown Developer | Fichiers divers | Unknown File Type | Unknown Software | Unknown Developer |
| .MV3 | Unknown Developer | Fichiers audio | Audio Data | Unknown Software | Unknown Developer |
| .MVO | Unknown Developer | Fichiers graphiques | A Graphic File | Unknown Software | Unknown Developer |
| .MWR | Unknown Developer | Fichiers divers | Unknown File Type | Unknown Software | Unknown Developer |
| .MWV | Unknown Developer | Fichiers de données | Media Works Data | Unknown Software | Unknown Developer |
| .MW_ | Unknown Developer | Fichiers audio | Audio Utility Midimach Compressed File | Unknown Software | Unknown Developer |
| .MYW | Unknown Developer | Fichiers d’image vectorielle | MyWorld Education Drawing | Unknown Software | Unknown Developer |
| .MZF | Unknown Developer | Fichiers compressés | MediaZip Archive | MediaZip | Unknown Developer |
| .M_S | Unknown Developer | Fichiers divers | Unknown File Type | Unknown Software | Unknown Developer |
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